Drifting above North America last autumn, a NASA satellite named Terra partook in some high-altitude leaf peeping. In an aerial photograph snapped in September, swaths of orange and red saturate the green landscape, as if igniting the planet in a smokeless blaze. If an alien spacefarer were to happen upon this annual scene, it might mistake the spectacle for an intentional signal—a battle flare, perhaps, or a distress call. Earth-bound scientists, on the other hand, long dismissed autumn colors as a mere coincidence. “Such biochemical extravagancies … seem to have no function in the trees,” wrote one plant physiologist in 2000, echoing the sentiment of the 19th-century Scottish biologist Marion Newbigin.

But in the late 1990s, William D. Hamilton, an evolutionary theorist at Oxford, proposed that trees’ vibrant fall hues were indeed a signal, meant to ward off parasites that lay their eggs in autumn. The behavior, he reasoned, must be an outcome of coevolution, in which two or more species reciprocally drive one another’s adaptations.